The idea of using electrochemical reaction between hydrogen and oxygen to generate electrical power started in 19th century; after that, scientists have continuously tried to put the technology of so-called “fuel cell” into practice and applied it in our daily life. Over the recent years, problems of scarce energy and environmental pollution caused from using fossil fuel have become more and more serious, a solution for energy problem demands for immediate attention. In view of this, the development of fuel cell technology will effectively relief energy problem that we confront at present. The fuel cell is a power-generating device with advantages of efficiency and cleanness; it is also an energy-transforming device capable of transforming chemical energy of fuel into electrical energy. From viewpoint of system efficiency, fuel cell has potential excellent advantages and its high efficient circulation with the combination of fuel cell and gas turbine is the utmost.
In recent year, due to various problems and pressures from environmental protection and insufficient energy, international organizations such as governments, vehicle industries, electrical power plants and energy industries have progressively focused on the development of fuel cell technology, where the same situation is encountered domestically. In view of these situations, the fuel cell technology can be addressed as one of the potential technology for clean energy in the future.
Up till now, the demonstrated products with application of Solid Oxide Fuel Cell (SOFC) as source of power has been introduced for almost a hundred years; yet during the evolving period of commercialization, lots of technical questions are still confronted and awaited for solution. Since 1960's, relevant companies kept developing the technology of tubular SOFC and SOFC power generating system, which could be seen as source of technical evolution reopened by the industries in recent history. During the mid of 1980's, fabrication technology of flat plate SOFC achieved an outbreak and made the cost of production more competitive than it was before. Therefore, most companies or research institutes (scattered around USA, Europe, Japan and Australia region) that devoted in research and development of SOFC technology at later stage focused on the flat plate system.
In the development of fuel cell over recent years, Proton Exchange Membrane Fuel Cell (PEMFC) and Direct Methanol Fuel Cell (DMFC) were specialized in the low temperature field, while Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC) were specialized in the high temperature field. Among them, the flat plate SOFC has higher potential and the reason besides advantages of high efficiency and low pollution for general fuel cell is that: (1) It has higher working temperature, which can perform CHP generation via heat technology and obtain over 80% of CHP efficiency. (2) The electrolyte is solid and has no problems such as evaporation, leakage and corrosion; thus the operational life is long and the design of appearance is more flexible. (3) By execution of electrochemical reaction under high temperature, the cell itself has internal ability of re-arrangement; thus it can directly use natural gas, coal gas or other hydrocarbons as fuel. (4) The remaining heat emitted is the unused fuel, which can combine with gas turbine to form a mixed system of circulative power generation and improve total efficiency of power generation, as well as reduction on environmental contamination. Since the operating temperature is higher however, requirements on material are stricter and the open circuit voltage is lower. At present, medium SOFC with working temperature between 600° C.-800° C. is under active international development to overcome the above-mentioned problem.
The SOFC mainly consists of an anode, cathode, solid oxide electrolyte and bipolar connecting plates (which are also named as “bipolar plates” or “interconnects”). The connecting plates are the most essential elements of SOFC, of which materials are mainly divided into ceramics and metals. The main function of connecting plate is to connect anode and cathode of adjacent cell, which also acts as physical barrier for protecting the recovered environment of air electrode material isolating fuel electrode terminal; as well as protecting oxidized environment of fuel electrode material isolating air electrode terminal.
The illustration of assembling 4-cell-stack is shown in FIG. 1 with a cell stack sealed with ceramic glass. There are some simplifications in the figure; firstly, we replace the shape of combination between Positive electrode—Electrolyte—Negative electrode plate (PEN) and the frame with a flat plate, so there is no problem of electrode connection, where mesh can be removed together. The fuel enters from inlet at top left side and then directly passes two connecting holes for fuel entrance on left side of top frame. Part of the fuel passes channel above the top connecting plate and exits from middle hole on right hand side of the connecting plate. Other part of fuel passes channel above second, third connecting plate and lower plate, which then connects lower right hand side of cell stack via middle hole on right hand side and finally exits. The air works in opposite manner; it enters from entrance on top right hand side, passes channel below upper plate, as well as first, second and third connecting plate to middle hole on right hand side of each frame, and then the air finally connects with draining pipe at lower left hand side of cell stack and exits. The center ventilation hole on right hand side of top frame must be closed for preventing fuel to backflow upwards and mix with the air. The ventilating holes on both sides of right hand side of bottom frame must also be closed to prevent air passes and direct mixes with the fuel.
Nowadays, metal connecting plates have become the main stream of SOFC, and these can be categorized into two major types, namely chromium base and iron base. Chromium base is the product at early stage; it allows higher operating temperature with higher cost, fabrication difficulty and worse ductility comparing to iron base. In view of this, the present trend of development is mainly based on iron base products. In addition, if the operating temperature of SOFC is lowered to 700° C., ferrite stainless steel can be used as material of connecting plate and this will dramatically reduces the cost.
Other than satisfying excellent material properties such as heat conductivity, electrical conductivity, corrosion resistance and oxidation or reduction resistance, the connecting plate still needs another important function, i.e. to guide fuel and air for even distribution in the reaction area. Whether the reacted gas can evenly flow within the channel or not will influence efficiency of power generation, and this is also the main factor for determining the distribution evenness of current density and temperature on the ceramic substrate. Therefore, the design of bipolar plate channel and the field analysis within the channel are very significant works in the process of development for fuel cell technology.
The U.S. Pat. No. 6,544,681, Apr. 8, 2003 (CORRUGATED FLOW FIELD PLATE ASSEMBLY FOR A FUEL CELL) illustrates a type of channel field with square wave shape channel, where the channel fields consist of two sides of a channel plate to provide channels for fuel and oxides respectively. Fluids on top and bottom sides may flow interactively via a coupling device and then generate electrical power with reaction. The above-mentioned patent focuses on interaction of the two fluids, which is clearly different from the object of the present invention to control the evenness of flow speed of the two fluids on the connecting plate. Furthermore, the U.S. Pat. No. 6,296,964, Oct. 2, 2001 (ENHANCED METHANOL UTILIZATION IN DIRECT METHANOL FUEL CELL) illustrates a type of channel field with square wave shape, which has a conducting plate at space of 90 degrees between each other; the plate is made with membrane-electrode and contacts with channels of both sides separately. The purpose is to control rate of methanol infiltration in the fuel cell and increase utilization efficiency of methanol, which is different from the object of the present invention, namely the focus on control even flow speed of the two fluids with connecting plate. In the present invention, interpretation was made by means of reverse flow (flow direction of fuel is opposite to air); since this patent focuses on channel design of connecting plate, the goal and scope of patent are different.
The Germany patent No. DE10039024A1 is a method of assembling SOFC stack and it is a cell stack sealed with glass ceramic. The flow direction is designed in co-flow fashion, i.e. the flow direction of fuel and air is consistent. The design of connecting plate uses ribs and grooves to form the channel field. However, it has not designed the number of inlets/outlets and there is no description on detail design of the flow area.
The structural design of connecting plate for general SOFC only considers number of inlets/outlets for the fluid, arrangement of location and design of geometric shape, where no detail consideration is given on even flow speed of working fluid within channel of connecting plate. In the design of connecting plate with internal manifold generally, the less the number of fluid inlets and outlets, the worse the even flow rate of fluid in each channel and vice versa, but the overall design of will be complicated and the production cost will increase. Therefore, the number of individual fluid inlets and outlets rarely exceeds three (including three). FIG. 2 shows the common SOFC design of channel for connecting plate with internal manifolds, which is the typical design of two inlets and one outlet. The fluid distribution in the channel is not very even; the flow rate of fluid at channel between two inlets is higher, but the flow rate of fluid at external channel on both sides is lower. In order to improve unevenness of flow speed, it is necessary to modify design of the channel to enhance overall efficiency of power generation with fuel cell.